On-Screen-Display messages play an important role in consumer electronics products by providing users with interactive information so as to guide them through the usage and configuration of the product. The increasing development of digital video technology presents an ever increasing problem of generating OSD messages which are compatible with the various commercially available digital video decoders. These decoders are generally designed to decode bitstreams with a specific format such as the ISO/IEC international Standards 11172 (1991) (generally referred to as MPEG-1 format) and 13818 (1995) (generally referred to as MPEG-2 format).
Although the MPEG standards specify a general coding methodology and syntax for generating a MPEG compliant bitstream, many variations are permitted, thereby supporting a broad range of applications such as desktop video publishing, video conferencing, digital storage media and television broadcast.
In effect, MPEG does not define a specific algorithm needed to produce a valid bitstream. Thus, MPEG encoder designers are accorded great flexibility in developing and implementing their own MPEG-specific algorithms.
This flexibility fosters development and implementation of various MPEG-specific algorithms. However, a common goal of MPEG encoder designers is to design a MPEG-specific algorithm that operates within a prescribed bit rate and operating delay constraint. As such, before an image is encoded, a MPEG-specific algorithm must execute a complex process of balancing numerous factors such as coding mode decisions, picture type, rate control, coarseness of the quantization scale, motion estimation, scalability, detection of distortion, effect of scene cuts and the nature of the signal within the macroblock. Without knowing in advance as to the nature of the images that are to be encoded, an encoder must spend many computational cycles to determine the optimal values for various parameters to properly encode an image.
In the field of OSD message generation, a frame of OSD messages is generally encoded in the same manner as other images without special treatment. However, unlike other images, OSD messages contain repetitively used components, i.e., the basic set of font characters (letters, numbers and other special characters). This prior knowledge of the nature of the input images can be used to improve the efficiency of the encoding method for a frame of OSD messages. Thus, a need exists for a method and apparatus for generating a valid bitstream of a frame of OSD messages by exploiting the repetitive nature of OSD messages.